Viscosity of ink is very important to a printing device that prints a large amount of printed matters. Since ink contains volatile solution in itself, its concentration tends to become higher as time goes. If viscosity of ink is changed as mentioned above, improvement of ink quality is not expected and a flow path in the printing device may be clogged due to the ink, causing malfunction of the device. Thus, most printing devices need a means for suitably keeping viscosity of ink within a predetermined range.
One of conventional ink viscosity measuring methods is depending much on a man ual work as shown in FIG. 1. That is to say, after ink is filled in a container 1 having a hole 2 of a certain size at its bottom, a time that the ink completely gets out of the container 1 through the hole 2 is measured to calculate viscosity of ink. However, this method requires too much time for measurement, and cannot ensure accurate measurement since the measurement should be continued for the last drop of ink. Most of all, since a worker measures viscosity of ink discontinuously and regulates it, the viscosity of ink cannot be maintained uniformly.
In addition, a method for measuring viscosity of ink by installing a mechanical measurement device is installed in the flow path of the ink has been proposed. However, in this case, the ink flow path of the measurement device is frequently clogged due to the ink having relatively greater viscosity, and also this method does not allow a worker to easily separate and clean the measurement device. Thus, a new measurement device should be frequently exchanged with an existing one, disadvantageously in terms of economy.